1. Field of the Invention
The present invention relates to an apparatus and a method for reducing a common mode voltage. More particularly, the present invention relates to an apparatus and a method that reduces common mode voltage in a drive motor whereby common mode noise attributed to capacitive coupling may be eliminated
2. Description of the Related Art
Hybrid vehicles, electric vehicles, and fuel cell vehicles, also known as green vehicles, commonly utilize an electric motor. Additionally, these vehicles include a high-voltage battery, which is designed to be repetitively charged and discharged during operation of vehicles, to provide power to the electric drive motor, and an inverter that feeds a driving force to the motor from the high-voltage battery, which drives the electric motor. The inverter is also used for regenerative braking (e.g., storing energy back into the battery from the electric drive motor). More particularly, the inverter subjects the high-voltage battery power to phase shift to drive the drive motor, but also acts as a power converter which uses the driving energy from the drive motor and the kinetic energy from regenerative braking to charge the high-voltage battery.
FIG. 1 is an exemplary schematic view of paths through which electromagnetic wave noise is diffused in green vehicles (e.g., environmentally friendly vehicles). The electromagnetic wave noise includes differential mode noise and common mode nose. When an inverter performs high-voltage and high-frequency switching for power conversion, differential mode noise is delivered (e.g., radiates) across the vehicle via high-voltage cables, drive motors and drive shafts. As for common mode noise, the capacitive coupling, attributed to the parasitic capacitance of high-voltage parts, generates common mode voltages, which induces common mode currents to flow. The common mode noise caused by the common mode voltage diffuses across the vehicle, which causes harmful effects on controllers within the vehicle (e.g., lowering the radio reception performance of the vehicle).
Further, FIG. 2 is an exemplary schematic view illustrating a mechanism that generates a common mode voltage in an electric drive motor, and the paths through which common mode noise is transmitted. When an inverter performs high-frequency switching for power conversion to generate a driving force for the electric drive motor, a common mode current flows, via a parasitic capacitance present between a stator and a rotor within the motor, between the rotor and a drive shaft, between the stator and a stator winder, between the stator and a motor body, between the motor body and a bearing, and between the bearing and the drive shaft, with the concomitant generation of a common mode voltage. This may be problematic since electromagnetic wave noise may be generated, which has a harmful effect on controllers (e.g., control units) within the vehicle and induces electrical corrosion on motor bearings, which causes noise and poor driving force transmission.